286 W. G. VanName — JEmhryology of Eustylochus. 



appears to have done with some degree of certainty. Neither 

 Klinckowstrom or Francotte give any evidence in support of this 

 conchision, upon which, nevertheless, they base their schemes of the 

 distribution of the chromatin in the egg and the polar bodies. 



Fig. 17 shows a very late stage of the second polar spindle, though 

 most of the second polar body, which is here already constricted off, 

 lies in another section. The chromosomes have swelled up into 

 vesicles, in which a few threads of a network are beginning to 

 appear, but as yet no nucleoli. The aster rays, centrosphere and 

 centrosome (it does not divide) have not yet begun to degenerate. 

 This is the last stage in which I have found this centrosome. 



The Female Pronucleus. 



The vesicles formed from the chromosomes remaining in the egg 

 after the second polar body has been expelled, increase in size and 

 fuse together to form^ a single large one, the female pronucleus, 

 which is at first irregularly lobed and elongated in a direction trans- 

 verse to the main axis of the Qgg. This fusion may occur while the 

 vesicles are still small, or it may be delayed till shortly before the 

 formation of the cleavage spindle. The following figures illustrate 

 the process: Figs. 17, 19, 20 and 21. 



The longer diameter of the pronucleus may exceed one-quarter of 

 that of the Qgg. In the vesicles which form it, and in the pronucleus 

 itself, there is a rather coarse network, which, however, does not 

 stain deeply during these stages. At the junctions of the fibers of 

 the network, and where they touch the nuclear membrane, are thick- 

 enings due apparently to accumulations of the substance composing 

 the fibers. In addition to the network, there are in the fulh-- formed 

 pronucleus several nucleoli of unequal size. One of these usually 

 originates in each vesicle, and as the latter become fused the nucleoli 

 also increase in size and unite, so that in the completed pronucleus 

 instead of ten nucleoli there are perhaps three or four quite large 

 ones. Their staining power, which is at first considerable, meanwhile 

 decreases so that they stain only with plasma stains. They are 

 spherical and bounded by a distinct outline. From the way they 

 fuse together they are probably of a viscous or semi-fluid nature in 

 the living egg. 



A short time before the membrane of the pronucleus begins to 

 disappear for the formation of the cleavage spindle the nucleoli 

 vanish. They are sometimes vacuolated before the time of dis- 



